Silicon used for a solar battery is generally required to have a 99.9999% or so purity. The various types of metal impurities are required to be 0.1 mass ppm or less and the B is required to be at least 0.3 mass ppm or less, preferably 0.1 mass ppm or less. As silicon satisfying this purity, there is semiconductor use silicon obtained by the Siemen's process, that is, high purity silicon obtained by distilling, then heat decomposing silicon chloride. However, this Siemen's process is high in cost and not suitable for solar batteries requiring large amounts of silicon.
As an inexpensive silicon, there is metal silicon obtained by using an arc furnace and reducing silica stone by carbon, but the purity is normally 98% or so and the result includes Fe, Al, Ca, or other various types of metal impurities and the P, B, etc. used as silicon dopants. For this reason, this cannot be used as is as the raw material for solar batteries. Therefore, many attempts have been made trying to refine this inexpensive metal silicon for use for solar batteries.
Among the impurities contained in metal silicon, Fe, Al, Ca, or other various types of metal impurities can be removed by the one-directional solidification method. That is, this refining method uses the phenomenon that when a silicon melt solidifies, the copresent molten silicon contains a large amount of metal impurities while the solidified silicon only takes in a little. “The concentration of impurities in solid phase silicon/concentration of impurities in liquid phase silicon” is called the “segregation coefficient”. With Fe, Al, Ca, and other various types of metal impurities, the segregation coefficient is far smaller than 1, so silicon can be removed by this one-directional solidification method. That is, by concentrating these impurities in the finally solidifying part, these impurities can be removed from the other major part of the silicon.
Further, the impurities with large vapor pressure in the metal silicon, for example, the P, Ca, Na, etc., can be removed from the silicon by melting the silicon under reduced pressure, that is, by the vacuum melting method.
As opposed to this, B has a segregation coefficient close to 1 and has a small vapor pressure, so is difficult to remove by the above methods. Various methods have therefore been proposed for this.
Japanese Unexamined Patent Publication No. 56-32319 discloses the method of acid washing silicon, the vacuum melting method, the one-direction solidification method, and a method of removal of B from silicon by the slag refining method. According to this, the extraction use melt, more specifically a total 10 kg of CaF2+CaO+SiO2 slag, and 5 kg of silicon can be melted together at 1450 to 1500° C. to reduce the boron B in the silicon from 30 mass ppm to 8 mass ppm. However, the rate of drop of B is small and the content of B remains large even after the treatment, so this is insufficient as silicon for use for solar batteries. Further, in the slag refining method, the B in the molten silicon is removed by being absorbed in the slag, but with slag of the above composition, the B distribution coefficient (concentration of B in molten slag/concentration of B in molten silicon) is a small 1.375, so there is the inconvenience that the slag refining method has to be repeated again and again. For example, when the concentration of B in the silicon is 10 mass ppm and using the above slag in double the amount of the silicon in the same way as the above example, even if the slag used contains no B at all, even if performing the slag refining operation three times, the B will not be reduced to 0.3 mass ppm or less. Further, normally, the slag used contains at least several mass ppm or so of B. Further, slag is normally used in only the same amount as the silicon or less, so the number of slag refining operations becomes further greater.
Japanese Unexamined Patent Publication No. 58-130114 describes a slag refining method comprising vigorously mixing slag containing one or both of an alkali earth metal or alkali metal oxide or slag ingredients and crushed crude silicon (purity equivalent to metal silicon) before melting, then melting them. However, in general, crushing the crude silicon of the raw material requires considerable cost. Further, at the time of crushing, contamination frequently occurs. Further, vigorous mixing also requires considerable cost. Further, when repeatedly performing this refining operation, it is necessary to crush the silicon each time and mix it with the slag. This becomes extremely troublesome. For this reason, in an industrial process, a processing including a crushing step and a mixing step is not preferable. Further, according to the examples in Japanese Unexamined Patent Publication No. 58-130114, the concentration of B in the finally obtained silicon is 1 mass ppm. This is insufficient as silicon used for solar batteries.
Japanese Unexamined Patent Publication No. 2003-12317 discloses a slag refining method adding flux (slag) to metal silicon and blowing in an oxidizing gas. In this method, it is considered possible to simultaneously realize a high basicity and high oxygen partial pressure of the slag and possible to efficiently remove the B in the silicon. As basic ingredients in the slag, CaO, CaCO3, and Na2O may be mentioned. The examples describe that B is reduced from an initial concentration of 14 mass ppm to 7.6 mass ppm. However, blowing gas into molten silicon is considerably difficult. In particular, there is no material suitable for forming the nozzle for blowing in the gas. Further, the concentration of B in the finally obtained silicon is also 7.6 mass ppm—which is insufficient as silicon used for solar batteries.
Further, “Distribution Behavior of Boron Between SiO2-saturated NaO0.5—CaO—SiO2 Flux-Molten Silicon” (Tanahashi et al., Shigen To Sozai (Journal of the Mining and Materials) Processing Institute of Japan, vol. 118, No. 7, P497 to 505, (2002)) also describes the slag refining method. The slag used is Na2O—CaO—SiO2. This slag is produced in advance at 1700° C. (1973K), then charged into a metal silicon bath with a high initial B concentration for slag refining. It is stated that the B distribution coefficient at this time is as high as 3.5 and that this is an improvement over the highest value of 2.2 of the B distribution coefficient up to then. However, with a B distribution coefficient of 3.5 or so, in principle, the concentration of B in the silicon can only be reduced to 0.4 mass ppm or so, so production of silicon for use for solar batteries is difficult. This is because, as explained later, the concentration of B in the slag used cannot be reduced to “zero” and 1 to several mass ppm or so is always included.
Further, as an industrial process in which slag refining is generally performed, there is the steelmaking process, but B oxides are far stabler than iron oxides, so in the steelmaking process, it is possible to oxidize the B without allowing oxidation of the iron and allow the B oxides formed to be absorbed by the slag and thereby easily remove them. As opposed to this, B oxides and silicon oxides are substantially the same in stability. If trying to oxidize B and get it absorbed by the slag, the silicon will also end up being oxidized. In this way, silicon and iron greatly differ in characteristics, so the slag refining technology of the steelmaking process cannot be applied as it is to silicon.
As a method for removal of B in silicon by a method other than slag refining, the method of oxidizing the B in the silicon and vaporizing it to remove it from the silicon has been conceived. However, due to the above mentioned reasons, silicon is also oxidized when oxidizing the B, so with each of the methods shown below, there is the problem of a low silicon recovery rate.
Japanese Unexamined Patent Publication No. 4-130009 discloses a method for advantageously removing B etc. by adding an H2O gas or O2, CO2, or other oxidizing gas and CaO, SiO2, or another oxygen-containing substance to a plasma gas. According to the examples, B is reduced from the initial 8.0 mass ppm to 0.2 mass ppm.
Japanese Unexamined Patent Publication No. 4-228414 also discloses the method of adding water vapor and silica (SiO2) to a plasma jet to refine silicon. According to the examples, B is reduced from the initial 17 mass ppm to 1.0 mass ppm.
Japanese Unexamined Patent Publication No. 5-246706 discloses a method of removal of B by generating an arc between molten silicon and an upper electrode and blowing an inert gas, preferably an oxidizing gas, into the vessel.
Further, as a method utilizing a special torch instead of plasma or an arc, U.S. Pat. No. 5,972,107 and U.S. Pat. No. 6,368,403 disclose a method of adding water vapor and SiO2 to an oxygen-hydrogen torch to refine molten silicon and further a method of adding, in addition to SiO2, CaO, BaO, and CaF2 to refine molten silicon.
As a method for removal of B as the oxidized gas and not using any plasma, arc, or special torch, Japanese Unexamined Patent Publication No. 4-193706 discloses a method of melting silicon in a container having a gas blowing tuyere at its bottom and mainly comprised of silica and further mixing and blowing Ar or H2 gas or a mixed gas of the same, preferably further with one or more types of oxidizing gas such as H2O, CO2, or O2 from this tuyere. In this method, the B can be considered to be removed in the form of an oxide gas. Further, it is described that when the raw material silicon has a high concentration of B, adding a mixture of one or more of SiO2, CaO, CaCl2, and CaF2 to the gas blown from the tuyere is advantageous for B removal. According to the examples, B is reduced from the initial 25 mass ppm to 5 mass ppm.
Japanese Unexamined Patent Publication No. 9-202611 discloses a method of removal of B comprising blowing one or more solids decomposing at 1400° C. or less and generating one or both of H2O or CO2 into a molten silicon bath together with a carrier gas. In this method, it is described that Ca(OH)2, CaCO3, and MgCO3 are used, and B becomes the oxide gas which is exhausted together with the carrier gas. Further, the examples describe that the B concentration in the silicon falls to 1 ppm or less.
WO89/02415 discloses a method comprising adding a chloride to cause the formation of boron chloride and thereby remove the B. For example, CaCl2, CaO, and SiO2 are used to lower the B concentration from the initial 17 mass ppm to 5 mass ppm.
The above conventional refining methods utilizing slag will be summarized next.
The first group comprises methods of absorbing and distributing B in the molten silicon in slag so as to lower the B in the silicon. These include the method of crushing and mixing silicon and the slag ingredients in advance before melting and the method of introducing an oxidizing gas in addition to the slag.
The second group comprises the methods utilizing plasma, an arc, or a special torch, adding an oxidizing gas or SiO2 or one or more of CaO, BaO, and CaF2 to convert the B to an oxide, and vaporizing it for removal.
The third group comprises the method of not using plasma etc., but blowing in Ar or H2, preferably one or more oxidizing gases of H2O, CO2, or O2, into the molten silicon, or the method of adding one or more of SiO2, CaO, CaCl2, and CaF2, and the method of blowing into the molten silicon bath one or more solids decomposing at 1400° C. or less and generating one or both of H2O or CO2 together with the carrier gas. Further, there is the method focusing on use of chlorides.